1. Field of the Invention
The present invention relates to line drivers for high-speed digital communications.
2. State of the Art
Broadband communications solutions, such as ADSL (Asynchronous Digital Subscriber Line) are increasingly in demand. The ability to achieve high data rates (e.g., 1 Mbps and above) between customer premises and the telephone system central office over existing (unconditioned) telephone lines poses a considerable technical challenge. ADSL is simply one example of a class of communications techniques generally known as Discrete Multitone (DMT). In DMT, a broad band of spectrum is divided into potentially a large number of sub-bands having a particular center frequency (tone). Conceptually, a digital communications signal is sent within each sub-band, all of the sub-band signals being added together to form a single complex signal. Referring to FIG. 1, in the specific case of ADSL, about 250 discrete tones are placed within a band from about 0.1 to 1.2 MHz (above the voice band, or POTS region), with information being modulated on each tone. Using such an arrangement, a combined bandwidth of up to 8 Mbps can be achieved.
An ADSL signal presents special problems for circuit designers because of its large xe2x80x9ccrest factor,xe2x80x9d defined as the ratio of the peak signal voltage to the average signal voltage. For ADSL, the crest factor is typically about 5.6. This high crest factor is accounted for by noting that, whereas the discrete multitone signals comprising an ADSL signal usually sum together to a relatively low average value, occasionally, the signals align in such as way as to sum together to a relatively high peak value. A typical ADSL signal might appear as shown in FIG. 2, for example, characterized by a high peak value, Vpk, and a low average value, Vave. The signal is produced by a digital signal processor (DSP) in the form of a digital signal having a sample rate in the range of about 3 to 5 MHz. (Also identified in FIG. 2 are voltage levels Vs1 and Vs2, referenced in the description that follows.)
FIG. 3 shows a first example of a prior-art ADSL line driver circuit. An 8 Mbps information stream is input to a DSP, which produces a complex modulated signal in the form of samples (e.g., 12 bit) at the aforementioned sample rate. The samples are converted to analog and the resulting signal low-pass filtered to produce a transmission signal. The transmission signal is applied to a linear (e.g., Class A) radio amplifier. Typically, the amplifier operates fromxc2x115 V power supplies and receives about 10 W of power. About 9.9 W of power are dissipated within the amplifier, while about 0.1 W of power is output through an isolation transformer to the phone line.
Because of the power consumption and heat dissipation of the circuit of FIG. 3, only a few hundred such line drivers can be accomodated within a typical central office of the telephone system. That is, only a very small percentage of the subscriber""s serviced by the central office can be provided with broadband service without rebuilding or reconfiguring the central office, which besides being very expensive is often not feasible.
FIG. 4 shows an improved prior-art ADSL line driver circuit. This circuit differs from that of FIG. 3 in that a xe2x80x9cClass Gxe2x80x9d amplifier is used, comprising the usual Class A amplifier in combination with a switchable power supply. During normal operation, a supply voltage select circuit is set to apply a lower supply voltage Vs2, sayxc2x15 V, to the amlifier. When greater signal range is required, the DSP produces a control signal PK that sets the supply voltage select circuit is set to apply a higher supply voltage Vs1 (e.g.,xc2x15 V) to the amlifier. The Class G amplifier dissipates about 3 W, as compared to almost 10 W in the original approach.
While the circuit of FIG. 4 provides a substantial improvement over the circuit of FIG. 3, the underlying problems of excessive power consumption, excessive heat dissipation, and low efficiency remain to a large degree.
Because of the linearity requirements of DMT/ADSL signals, circuits used heretofore for DMT/ADSL line drivers have been linear circuits. Other existing non-linear, switch-mode circuits have generally not been considered for this purpose. One such circuit is the sigma-delta modulator. Sigma-delta modulators are widely used for data conversion. A sigma-delta converter modulates a varying-amplitude analog input signal into a simple digital code at a frequency much higher than the Nyquist rate.
Another non-linear prior-art circuit is the Class D amplifier, most commonly used for audio applications. In a Class D amplifier, an input signal (typically an audio signal) is transformed into an output signal capable of being reproduced into the original signal on an external load, usually a speaker. In the basic operation of a Class D amplifier, an incoming signal is converted by a pulse-width modulator into a high-frequency rectangular wave, the average value of which tracks the original signal. The rectangular wave is fed into an output stage which provides level shifting and splits the signal into a driving signal high/low driving signals. The driving signals are filtered to remove switching noise, providing an averaged output to drive a load such as a speaker. Within the output stage, however, the high/low signals result in significant distortion due to imperfectly matched components. More particularly, pulses produced by pull-up and pull-down transistors, respectively, exhibit substantially different shapes. Such distortion is unacceptable in applications such as ADSL.
There remains a need for a line driver circuit for high crest-factor signals such as DMT/ADSL signals that consumes less power, dissipates less heat and is more efficient.
The present invention, generally speaking, achieves a highly efficient line driver for high crest-factor signals such as DMT/ADSL signals. In an exemplary embodiment, a digital signal produced by a digital signal processor or the like is processed by a sigma-delta modulator to produce one or more binary signal pairs. The signals of a signal pair are low-pass filtered, if necessary, and applied across the winding of a transformer. The transformer has a single secondary winding connected to the line and may has as many primary windings as the number of signal pairs. The transformer may have a unity turns ratio or may have a turns ratio for accomplishing voltage step-up. For one signal pair, the number of possible resulting signals levels on the secondary side is three, for two signal pairs five, etc. Using more than two signal levels, it becomes possible to recreate from the digital signals the corresponding analog waveform with the required accuracy. The circuit requires only a single supply voltage, is inherently balanced and provides high-voltage DC isolation.